1. Field of the Invention
This invention relates generally to direct access storage devices and, more particularly, to control of arm movement in disk drive devices.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk, for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from and recorded onto the disk surface. A read/write head produces and detects variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a path centered over the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data. Positioning read/write heads is one of the most critical aspects of recording and retrieving data in disk storage systems. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of data that a disk drive customer wants to record or read. Accordingly, a great deal of effort is devoted to servo control systems.
A servo control system generally maintains a read/write head in a position centered over a track by reading servo information recorded on the disk surface. The servo information comprises a position-encoded servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in disk servo tracks. The flux transitions are recorded as periodic servo pattern bursts formed as parallel stripes in the servo tracks. When the read/write head passes over the servo pattern flux transitions, the head generates an analog signal whose repeating cyclic variations can be demodulated and decoded to indicate the position of the head over the disk. The position indicating information can be used to produce a corrective signal that is referred to as a position error sensing (PES) signal. The PES signal indicates which direction the head should be moved to remain centered over a selected track and properly read and write data.
In the sector servo method of providing servo track information, each disk surface is divided into angularly-spaced sectors, with each sector containing both pre-recorded servo track information and customer data. Typically, the tracks on a sector servo disk surface are partitioned by having a short servo track information area followed by a customer data area. The servo track information area typically includes a sector identification marker, track identification data, and a servo burst pattern. The sector marker indicates to the data read/write head that servo information immediately follows in the track. In a sector servo system, the servo read head is typically the same head used for reading data.
Some servo control systems include an estimator that predicts the velocity and position of the disk arm actuator during a servo control signal computation interval, based on the PES signal. The estimator implements a state estimator function that includes constant terms for system parameters such as drive motor torque factor, disk arm inertia, computation delay, arm pivot-to-head distance, sample time, and the like, along with variable terms for PES signal gain, PES sampling time, and the like. Typically, the magnitude of the constant terms is determined by the design of the disk drive system and their value is never altered during the calculation of the estimator function. The estimator is often implemented as part of the servo controller.
One of the estimator function constant terms is a time value for sampling of the PES signal. That is, conventional estimator design assumes that the PES signal sampling is regularly spaced in time. Under certain conditions, this may not be true. For example, the drive motor that rotates disks may have a speed accuracy tolerance of plus or minus 3%. A variation in drive motor speed results in a variation in the rate at which PES samples are received. If a constant PES sampling time value is assumed in the estimator function, and the sampling time is changing, then the estimator output will produce a servo control signal that may be erroneous whenever the drive motor speed varies from its nominal value.
It is known to use an actual PES sampling time for one servo control signal computation interval to modify the estimator function constants for subsequent computation intervals. Such a system is described, for example, in U.S. Pat. No. 4,816,941 to S. Edel and I. Van Pham. In the '941 patent, certain estimator function constant terms that are derived from the PES sampling time are modified with the actual PES sampling time value from a preceding computation interval. In this way, these particular constant terms of the estimator function are updated with actual PES sampling time data. This type of modification to the estimator function is especially suited to the situation where the PES sampling time interval is changing because the drive motor speed is varying, because in this situation, the change in one sample time period is related to the change in the very next sample time period.
There may be other situations in which the PES sampling times are not regular, and in which the change in one sample time interval is not related to the change in the next sample time interval. For example, in a multiple-disk, multiple-head disk drive system, a read or write operation may require a head switch, moving from a head that is over one disk surface to a head that is over a different disk surface. If the respective disk sectors are not aligned in time from surface to surface, then consecutive PES signal sample times will not be uniform. Another situation in which PES signal samples may not be uniform is for a disk drive with a sample rate that varies from disk surface to disk surface. Irregular PES samples also may occur with a disk drive in which the servo sectors are not radially aligned. In these situations, a prior PES sample time interval may be unrelated to the subsequent PES sample time interval. Thus, different estimator constants may be affected and the technique described in the '941 patent may not be suitable.
Each situation in which the PES sample time is different from the expected uniform or regular sample time is called an "odd" sample time. Even one odd sample time can result in a misprediction of disk arm velocity and position, and can be extremely deleterious if it happens while the actuator arm is moving at a high velocity.
From the discussion above, it should be apparent that there is a need for a disk drive servo control estimator that can provide velocity and position predictions with improved accuracy, considering odd sample times. The present invention fulfills this need.